Buck-Boost converters are used in applications where the desired output voltage (e.g. 3.3V) can be lower or higher than the input voltage (e.g. Vin=2.5V-5.5V). FIG. 1 prior art gives an example of a typical buck-boost switch configuration. This configuration is called non-inverting or positive buck-boost converter as the output voltage has the same sign as the input voltage.
In basic operating principle referring to FIG. 1 prior art, when both the switches 10 and 11 are in phase-1, the inductor L is connected to supplies and is charged with current and, when both the switches 10 and 11 are in phase-2, the inductor current IL charges the output capacitor 12. The output voltage V versus input voltage Vg as a function of duty cycle in this mode of operation is given by the equation (1):
                              V                      V            g                          =                  D                      1            -            D                                              (        1        )            
Thus the converter is capable of achieving output voltages lower or higher than the input voltage.
FIG. 2 prior art gives the topology of an output stage of a synchronous buck-boost controller with integrated switches.
Referring back to basic operation of FIG. 1 prior art, in phase-1 switches M1 and M3 will be ON; switches M2 and M4 will be OFF; and in phase-2 switches M2 and M4 will be ON and switches M1 and M3 will be OFF.
Comparing the basic Buck-Boost operation with a typical Buck or Boost converter, we can list the following disadvantages:                4 switches change state at each cycle, thus switching loss is 2 times that of a typical Buck or Boost converter        The average inductance current is significantly higher than the load current, given as:IL=ILOAD/(1−D)(e.g. when D=0.5, Vin=Vout, IL=2×ILOAD),        which leads to                    an increase in inductor current            increase in resistive losses (e.g. for D=0.5 example, losses due to the equivalent series resistance of the inductor (RESR,L) will be 4 times that of a Buck converter.)                        At phase-1 only the capacitor is sourcing the load, thus a low equivalent series resistance (ESR) of the capacitor is needed.        Higher current ripple on the inductor.        
The mentioned disadvantages can be reduced if we use separated Buck and Boost pulses, which means in a given cycle, either Buck switches M1 and M2 OR Boost switches M3 and M4 will be switching. During Buck mode switching M3 will be OFF and M4 will be ON, likewise during Boost mode switching M1 will be ON and M2 will be OFF.
An example of a Buck-Boost converter utilizing “separated Buck and Boost pulses” using voltage mode control is disclosed by U.S. Pat. No. 6,166,527 (to Trevor W. Barcelo). FIG. 3 prior art shows this implementation, wherein the control voltage 30 is compared with two adjacent saw tooth signals wherein boost signal 31 is fed into boost comparator 34 and buck signal 32 is fed into buck comparator 33.
If the control voltage 30 is low, it will only be crossing the buck saw tooth signal 32 and only Buck switching will occur. Similarly if the control voltage is high, it will only be crossing the boost saw tooth signal 31 and only Boost switching will occur. An example of switching versus changing control voltage is given by FIG. 4a illustrating PWM pulse generation for buck and boost operation using one control signal Vctrl 40 and two saw tooth signals.
The following parameters are used in FIGS. 4a and 4b: 
G1 is the buck switch control signal, which becomes low when the buck sawtooth crosses control voltage 40.
G2 is the boost switch control signal, which becomes low when the boost sawtooth crosses control voltage 42.
VLx and VHx are typically defined by design specifications—e.g. input voltage range.
A similar technique is using two-shifted control voltages and a single saw tooth signal, as illustrated in FIG. 4b illustrating PMW pulse generation for buck and boost operation using two control signals Vbuck_ctrl 41 and Vboost_ctrl 42 and a single saw tooth signal. When the Vbuck_ctrl 41 control signal crosses the saw tooth signal, Buck switching will occur, similarly when the Vboost_ctrl 42 control signal crosses the saw tooth signal, Boost switching takes place. In FIG. 4b the amplitude of the saw tooth signal is equal to the control voltage shift. Thus in a given cycle either a Buck or Boost switching will occur.
As in Buck or Boost converters, current mode control (CMC) can also be used in Buck-Boost converters. And though CMC is more desirable for most applications, it has serious challenges if separated Buck and Boost pulses are required.
There have been examples of current mode control (CMC) Buck-Boost converters utilizing separated Buck and Boost pulses as disclosed in (Ma, Yanzhao, Jun Cheng, and Guican Chen. “A high efficiency current mode step-up/step-down DC-DC converter with smooth transition.” In ASIC (ASICON), 2011 IEEE 9th International Conference on, pp. 108-111. IEEE, 2011) and in (Ehrhart, Andreas, Bernhard Wicht, Moris Lin, Yung-Sheng Huang, Yu-Huei Lee, and Ke-Horng Chen “Adaptive Pulse Skipping and Adaptive Compensation Capacitance Techniques in Current-Mode Buck-Boost DC-DC Converters for Fast Transient Response”).
In both examples above the decision on staying in the Buck region or in the Boost region is given by checking the duty cycle as e.g. in the Ehrhardt et al. publication changing from Buck to Boost mode is decided when DBuck>90% (in the next switching cycle, Boost switching will take place), and changing from Boost to Buck mode is decided when DBoost<10%.
There is a serious disadvantage of changing modes with this technique, for example when changing from Buck mode to Boost mode, it will take significant time for the loop filter to settle to normal operation, which results in unacceptable output transients.
Furthermore another severe problem of prior art buck-boost converters is output voltage overshoot after mode changes.